Traditional broadcasting networks, such as analog telecast systems, are Multi-Frequency Networks (MFN). These networks operate several transmitters each on different frequencies in different service areas to avoid interfering with one another. In MFN mode, a TV channel requires a large amount of frequency bandwidth to reach a large coverage and thus results in low spectral efficiency.
As spectrum resources become scarce, Single Frequency Network (SFN) becomes a hot spot. An SFN is created when all the transmitters in a network service area operate on the same carrier frequency. In an SFN, multiple transmitters over an area simultaneously transmit identical signals (simulcast) on the same physical resource (time, resource block). From the receiver (for example, the terminal) point of view, the received signal is indistinguishable from a single (cell) transmission. Delayed versions of the signal due to the multi-cell transmission are observed at the receiver. These delayed versions may be treated as multi-path components of the signal and can be combined in the receiver. By doing so, inter-cell interference can be transformed into useful signal energy, and the distribution of carrier to interference ratio across the coverage area is improved.
SFNs are primarily defined in terms of coverage areas (SFN areas), i.e., the set of cells that are participation in the simulcast transmissions. The cells and content in each SFN need to be tightly synchronized and coordinated.
SFN can work with multi-carrier modulation technique such as Orthogonal Frequency Division Multiplexing (OFDM). OFDM has been introduced into the Long Term Evolution (LTE) of 3GPP standards. The frame structure of LTE TDD (time division duplex) mode is similar to that of TD-SCDMA.
In CDMA systems operating in the SFN mode, signals from different base stations may be treated as multi-path components. This leads to a higher demand on mobile receivers to tolerate a higher delay spread of multi-path signals whose power levels are similar to that of the main-path signal. As a result, the time synchronization and the transmission synchronization among different base stations can have a significant impact on the receiving ability of mobile receivers.
Originally, streaming media is the transmission method for transmitting television programs over a mobile communication system. Compared to 2.5G techniques, 3G techniques provide higher data rate and support for higher spectral efficiency. 3G brings new chances to develop mobile television.
MBMS (Multimedia Broadcast Multicast Service) is introduced in 3GPP R6 in order to provide more multimedia services than streaming media. The main evolution of MBMS include a new network element of BM-SC, upgrade of existing network elements of PS domain for new MBMS interfaces (such as Gmb), new Channels (such as MICH, MTCH/MCCH/MSCH), new physical procedures (such as FACH channel selection combining, PTM and PTP handover), and new service procedure (such as subscription).
On the side of user equipment (UE), MBMS inherits from existing 3GPP standards as much as possible, except for a higher processing ability of base band.
TD-SCDMA is a part of 3G standards, which is an N-frequency system, i.e., a type of multi-carrier system. The number of carriers is N in a cell, and one of these carriers is the primary frequency, and the others (number of N−1) are secondary frequencies. The frequency used by UE is named the working frequency.
FIG. 1 shows a simplified example of a typical mobile communication system. The system has cells 1001-100Z (100), each cell composed of a NodeB (Base Station) 1011-101Z (101), and a number of UE 1021-102K (102). Each UE 102 connects with NodeB 101 in a serving cell 100 by a radio channel to communicate with other network elements. The direction from UE 102 to NodeB 101 is named the uplink, and from NodeB 101 to UE 102 is named the downlink. NodeB 101 is controlled by RNC (Radio Network Controller) 103. Together, NodeB 101, RNC 103, and some other network elements constitute the UTRAN (U MTS Terrestrial Radio Access Network) 110.
FIG. 2 shows the frame structure of a TD-SCDMA system. The structure is specified by 3GPP Technical Specification 25.221. The chip rate of TD-SCDMA is 1.28 Mcps. The time duration of each Radio Frame 200 is 10 ms identically divided into two sub-frames, 2010-2011, (201). Time duration of each sub-frame is 5 ms, i.e., 6400 chips. A sub-frame includes 7 Timeslots (from TS0 to TS6) 2020-2026, two pilot timeslots that are downlink pilot timeslot (DwPTS) 203 and uplink pilot timeslot 205, and a guard period (GP) 204. Furthermore, TS0 2020 is used for downlink that carries only system broadcast channel and other downlink traffic channels. The six timeslots, from TS1 to TS6 2021-2026, are used to transmit downlink and uplink traffic channels. UpPTS 205 and DwPTS 203 are used for acquisitions of uplink and downlink synchronization, respectively. Time duration of these timeslots, from TS0 to TS6 2020-2026, are 0.675 ms, i.e., 864 chips. Each timeslot includes two data parts (Data Part 1 208 and Data Part 2 210), and a 144-chip long training sequence (midamble 209). Midamble 209 is important for channel estimation, cell identification, and other procedures in TS-SCDMA. DwPTS 203 includes a 32 chip long GP 211 and a 64 chip long downlink sync code (SYNC-DL) 206. UpPTS 205 includes a 128 chip long uplink sync code (SYNC-UL) 207. There is a switch point 212 in the six timeslots from TS1 to TS6 2021-2026 and is located between TS3 2023 and TS4 2024 when there are 3 uplink timeslots and 3 downlink timeslots.
In an N-Frequency TD-SCDMA system, over Secondary frequencies, TS0 2020 and DwPTS 203 aren't transmitted in order to reduce co-channel interference between adjacent cells because of the omni transmission of TS0 and DwPTS signals without beam-forming.
FIG. 3 shows an example of a typical N-Frequency networking schematic, which is a 5 MHz networking with N equal to 3. There are 3 carriers in each cell, and one is the primary frequency 301, and the other two are secondary frequencies (302 and 303). TS1, TS2 and TS3 (2021-2023) are uplink timeslots; TS4, TS5 and TS6 (2024-2026) are downlink timeslots. Furthermore, the uplink/downlink ratio is configurable.
In an N-frequency scheme, in order to reduce co-channel interference, secondary frequencies (302 and 303) do not transmit DwPTS 203 and do not transmit pilot channel over TS0 2020. Frequency planning should be used in order to avoid adjacent cells having the same primary frequency. Techniques such as frequency planning, antenna locating, and sector area dividing should may be used. The main frequency of the adjacent areas should be different.
In TD-SCDMA, smart antenna technique is important. The transmission methods of signal, such as omni transmission of broadcasting or transmission with beam-forming, can lead to different networking methods.
In existing 3GPP or TD-SCDMA technical specification, according to the status whether it is same as the working frequency of UE, a frequency is categorized as intra-frequency or inter-frequency. In the messages that UTRAN sends to UE, such as Measurement Control or System Information message, there are two frequency measurement lists—“Intra-frequency cell info list” and “Inter-frequency cell info list. Each element in these lists indicates “cell info” information element (IE). In addition to IEs in “Intra-frequency cell info list”, elements in “Inter-frequency cell info list” includes a “Frequency info” IE. Therefore, the corresponding frequency of all “cell info” IE in “Intra-frequency cell info list” is the same as the working frequency of UE. There is no frequency information in “cell info” IE.
FIG. 4 shows an example of UTRAN sending the information of serving cell and neighboring cells to UE in different states. UTRAN 110 builds a SYSTEM INFORMATION 401 message and broadcasts the message. UE 102 receives this message in idle mode, and then updates local stored information according to the received message; if necessary, UE 102 can measure the Measurement Quantity specified by the message. UTRAN 110 also sends MEASUREMENT CONTROL 402 message to offer information of serving and neighboring cells to UE 102 in connection mode.
The SYSTEM INFORMATION 401 message and MEASUREMENT CONTROL 402 message include measurement object list, measurement quantity, and other information. Such information is classified into inter-frequency list and intra-frequency list. Tables 1˜4 show some common IEs in the two messages in existing 3GPP and TD-SCDMA technical specifications.
Table 1 and Table 2 show parts of “Intra-frequency cell info list” IE 30 and “Inter-Frequency cell info list” IE 40, which inform UE 102 of the objects list of intra-frequency and inter-frequency, respectively.
TABLE 1Parts of Intra-frequency cell info list IEInformationElement/GroupSemanticsName 30MultiType and referencedescriptionNew intra-1 tofrequency<maxCellMeas>cells 31>Intra-Integerfrequency(0 . . . <maxCellMeas>−1)cell id 32>Cell info 33
TABLE 2Parts of Inter-frequency cell info list IEInformationElement/GroupSemanticsName 40MultiType and referencedescriptionNew inter-1 tofrequency<maxCellMeas>cells 41>Inter-Integerfrequency(0..<maxCellMeas>−1)cell id 42>Frequencyinfo 43>Cell info 33
IE 30 is a structure array whose dimension is identified by “New intra-frequency cells” IE 31. Each element in the array includes two IEs, “Intra-frequency cell id” IE 32 and “Cell info” IE 33. The structure of IE 40 is similar to IE 30 while the main differences between them is that there is a “Frequency info” 43 IE in IE 40.
As to TD-SCDMA system, value of “maxCellMeas” equals 32. Table 3 shows the TDD parts of “Cell Info” IE 33 that pertains to cell information. “Primary CCPCH info” IE 51 provides information of P-CCPCH channel; “Timeslot list” IE 52 provides information of timeslot count maybe be measured by UE; “Timeslot number” IE 53 then indicates which timeslot should be measured.
Table 4 shows TDD parts of “Primary CCPCH info” IE 51. “Cell parameters ID” IE 61 provides identification information of primary frequency of a cell. The identification information is important for UE to distinguish the basic Mid-amble code and scramble code of a cell.
TABLE 3TDD parts of Cell info IEInformation Element/GroupType andSemanticsName 33Multireferencedescription> Primary CCPCH info 51> Primary CCPCH Tx power>Timeslot list 521 to <max TS.>>Timeslot number 53Integer(0 . . . 6)
TABLE 4TDD parts of Primary CCPCH info IEInformation Element/GroupSemanticsName 51MultiType and referencedescription>>Cell parameters ID 61Integer(0 . . . 127)
In TD-SCDMA systems, the following technical problems of applying SFN mode might be encountered:
The timeslot of a cell in SFN mode is the same as that of its neighboring cells. Thus, the timeslot should be transmitted unidirectionally and without beam-forming. Therefore, the same timeslot of neighboring cells could not work on an N-frequency mode for co-channel interference.
The “Cell parameters ID” of a timeslot in SFN mode configured by a cell must be the same as its neighboring cells. And the other timeslots of this cell working on N-frequency mode should apply cell-specific “Cell parameters ID”. Thus, there are maybe two or more different “Cell parameters ID” in the same cell. However, a “Primary CCPCH info” IE 51 could only contain one “Cell parameters ID” IE 61.
UE could not distinguish if a timeslot is in SFN mode or in N-frequency mode.
In summary, existing N-frequency scheme of TD-SCDMA is not suitable for mobile TV services. SFN scheme can provide higher spectral efficiency. And existing messages between UTRAN and UE may not be suitable for information transmission of SFN configuration.